3rd Generation Partnership Project Long Term Evolution (3GPP LTE) adopts Orthogonal Frequency Division Multiple Access (OFDMA) as a downlink communication scheme from a base station (may be referred to as “eNB”) to a terminal (may be referred to as “UE” (User Equipment)) and also adopts a Single Carrier-Frequency Division Multiple Access (SC-FDMA) as an uplink communication scheme from a terminal to a base station (e.g., see Non-Patent Literature (hereinafter, referred to as “NPL”) 1 to NPL 3).
In LTE, base stations allocate resource blocks (RBs) in a system band to terminals for every time-unit called “subframe” to perform communication. FIG. 1 illustrates a subframe configuration example in an uplink shared channel (Physical Uplink Shared Channel: PUSCH). As illustrated in FIG. 1, one subframe consists of two time slots. In each slot, multiple SC-FDMA data symbols and a demodulation reference signal (DMRS) are time-multiplexed. Upon receiving PUSCH, the base station performs channel estimation using DMRS. The base station then demodulates and decodes the SC-FDMA data symbols using the channel estimate.
Meanwhile, Machine-to-Machine (M2M) communication has been considered a promising technique for an infrastructure to support the future information society in recent years. The M2M communication enables service using inter-device autonomous communication without involving user's judgment. “Smart grid” may be a specific application example of the M2M communication system. The smart grid is an infrastructure system that efficiently supplies a lifeline such as electricity or gas, and performs M2M communication between a smart meter provided in each home or building and a central server, and autonomously and effectively brings supply and demand for resources into balance. Other application examples of the M2M communication system include a monitoring system for goods management or remote medical care, or remote inventory or charge management of vending machines.
In M2M communication systems, use of a cellular system having a broad range of a communication area in particular is attracting attention. In 3GPP, studies on M2M to be used in such a cellular network have been carried out in LTE and LTE-Advanced standardization under the title of “Machine Type Communication (MTC).” In particular, studies on “Coverage Enhancement,” which further expands the communication area, have been carried out in order to support situations where an MTC communication device such as a smart meter is installed at a location where the device cannot be used in the existing communication area, such as the basement of a building (e.g., see NPL 4).
In the MTC coverage enhancement, in particular, a technique called “repetition,” which repeatedly transmits the same signal multiple times, is considered an important technique for expanding the communication area. More specifically, performing repetition transmission on PUSCH has been discussed. The base stations, which are the receiver side of PUSCH, can attempt to improve the received signal power by combining the signals transmitted by repetition transmission and thus can expand the communication area.
The repetition transmission requires a large number of time resources for transmission of the same signal and thus causes degradation of spectral efficiency. For this reason, it is desirable to reduce the number of repetitions required for achieving a required coverage enhancement, as much as possible. In this respect, studies have been carried out on techniques for reducing the number of repetitions required for achieving a required coverage enhancement on PUSCH. Examples of the techniques for reducing the number of repetitions required for achieving a required coverage enhancement include “multiple subframe channel estimation and symbol level combining” (e.g., see NPL 5).
In multiple subframe channel estimation and symbol level combining, the base station performs coherent combining on a per-symbol basis over the number of subframes (NSF subframes) equal to or smaller than the number of repetitions for the signals transmitted by repetition transmission over multiple subframes (NRep subframes) as illustrated in FIG. 2. The base station then performs channel estimation using the DMRS after the coherent combining and demodulates and decodes SC-FDMA data symbols using the obtained channel estimate.
When the number of subframes (NSF), which is the unit for multiple subframe channel estimation and symbol level combining, is smaller than the number of repetitions (NRep), the base station combines the modulated and decoded symbols (NRep/NSF).
It has already become obvious that the use of multiple subframe channel estimation and symbol level combining can improve the transmission quality of PUSCH compared with plain repetition that performs channel estimation and demodulation and decoding of SC-FDMA data symbols on a per-subframe basis (e.g., see NPL 5). For example, in multiple subframe channel estimation and symbol level combining with four subframes (NSF=4), Signal to Noise power Ratio (SNR) required for achieving a required Block Error Ratio (BLER) can be improved by 1.4 to 2.6 dB compared with plain repetition. In addition, in multiple subframe channel estimation and symbol level combining with eight subframes (NSF=8), SNR required for achieving a required BLER can be improved by 1.9 to 3.5 dB compared with plain repetition.
In order to prevent degradation of channel estimation accuracy in PUSCH repetition, as illustrated in FIG. 3, increasing the number of symbols within which DMRS is inserted with respect to the existing DMRS symbols (see upper part of FIG. 3) in PUSCH has been proposed (see lower part of FIG. 3, and also see NPL 6, for example). Increasing the number of DMRS symbols results in an increase in the number of DMRSs (i.e., DMRS density) available for channel estimation and symbol level combining and thus is effective in improving the channel estimation accuracy.